WASHINGTON — The front-line regulator charged with overseeing JPMorgan Chase conceded on Wednesday that his agency stumbled when it overlooked a multibillion-dollar trading loss that has damaged the bank’s image and stock price.

At a Senate Banking Committee hearing, the comptroller of the currency, Thomas J. Curry, faced the brunt of the scrutiny, including a fiery round of questions from Senator Sherrod Brown. The Ohio Democrat asked whether the agency met a broad “standard that it set for itself.” Mr. Curry replied, “No, not in this particular case.”

“We would expect to be aware of significant risks,” he said in his first Congressional testimony since taking the helm of the comptroller’s office in April. Mr. Curry disclosed that the agency was conducting “a critical self-review,” which he said he hoped would be completed in the next several weeks.

Lawmakers are building to the much-awaited appearance next week of JPMorgan’s chief executive, Jamie Dimon, who is expected to shed new light on the bank’s trading blunder. Mr. Dimon is also scheduled to testify before the House Financial Services Committee later in June.

“While the JPMorgan trading loss does not appear to have caused systemic problems, it is a clear reminder that Wall Street continues to need better risk management, vigorous oversight and, if the rules are broken, unyielding enforcement,” said Senator Tim Johnson, the South Dakota Democrat who leads the banking committee.

The hearing on Wednesday gave lawmakers a fresh opportunity to inveigh against Wall Street risk-taking. Several Democrats have seized on the news of the bank’s loss, saying the case underscores a need to enforce a strict Volcker Rule.

The rule, named for Paul A. Volcker, the former chairman of the Federal Reserve, would ban banks from trading with their own money, a practice known as proprietary trading. Support for the new regulations gained momentum after JPMorgan’s loss disclosure last month.

But the scope of the rule, which regulators plan to complete in the coming months, is unclear. For one, it allows banks to use hedges to offset risk. Regulators have yet to decide how broad to make that hedging exemption, prompting some Democrats to push for clarity.

“Are you going to support closing the loopholes?” asked Senator Jeff Merkley, an Oregon Democrat and a member of the Senate Banking Committee who has championed the Volcker Rule.

Regulators would not be pinned down, saying it was too soon to tell exactly what went wrong at JPMorgan. But they did acknowledge that the JPMorgan loss would weigh on their deliberations when shaping the Volcker Rule.

“I would think our experience here with JPMorgan Chase would inform our views in the final rule-making,” Mr. Curry said.

Lawmakers also questioned regulators on the nuances of JPMorgan’s trading, asking whether the Volcker Rule would have outlawed JPMorgan’s positions.

With the line still blurred between hedging and proprietary trading, there was no clear answer. At the outset, bank officials said they were hedging their broad exposure to the markets. But as JPMorgan’s position shifted and grew, the trades apparently turned into a risky proprietary bet.

Neal S. Wolin, deputy secretary of the Treasury, called the JPMorgan incident “an important input” in creating a “strong Volcker Rule.”

Republicans, however, warned against overzealous regulation. “Some have used JPMorgan’s loss as an opportunity to argue for a stronger implementation of the Volcker Rule,” said Senator Richard Shelby of Alabama, the ranking Republican on the banking committee. That position, he said, was “a bit premature.”

Lawmakers, who occasionally digressed to express concern about the European debt crisis, saved the fireworks for Mr. Curry.

“The O.C.C. has a well-deserved reputation for being too cozy with the banks,” he said. “What I don’t want to see is a repeat of 2008,” when Wall Street risk-taking led to a taxpayer-financed bailout.

In the case of JPMorgan, some Democrats said that of the 65 examiners from the comptroller’s office assigned to the bank’s Manhattan headquarters, none kept a daily watch over the chief investment office, which was responsible for the trades. And the agency has only five officials stationed in London, where the trades were executed. Those examiners must keep an eye on a half-dozen banks, Mr. Curry said on Wednesday.

“We are assessing the adequacy of risk management throughout the bank,” Mr. Curry said. He added that the agency was examining whether JPMorgan provided regulators with sufficient information about the trades.

Despite lawmakers’ attacks on Wednesday, some noted that no one could remove all risk from Wall Street.

“It’s a fool’s errand to think regulators are going to be ahead of bankers,” said Senator Bob Corker, Republican of Tennessee.8:51 p.m. | Updated

A version of this article appears in print on 06/07/2012, on page B5 of the NewYork edition with the headline: U.S. Regulator Concedes Oversight Lapse in JPMorgan Loss.